APT Signing Key for SpiderOak

Beginning with SpiderOakONE 6.1.4, our APT packages (for Debian, Ubuntu, Mint, and similar Linux distributions) are now signed with the more secure SHA256. This is for improved security, but it does mean that their public key has changed and you will need to add that key in order to install our packages without a warning such as:

W: GPG ERROR: HTTP://APT.SPIDEROAK.COM RELEASE INRELEASE: THE FOLLOWING SIGNATURES COULDN'T BE VERIFIED BECAUSE THE PUBLIC KEY IS NOT AVAILABLE: NO_PUBKEY 573E3D1C51AE1B3D. REQUIRES INSTALLATION OF UNTRUSTED PACKAGES THIS REQUIRES INSTALLING PACKAGES FROM UNAUTHENTICATED SOURCES